1. Area of the Art
The invention relates generally to the field of manufacturing semiconductor single crystal, and specifically to the Czochralski (CZ) method of growing silicon single crystal of a large diameter with improved mechanical strength of the neck section of the silicon single crystal.
2. Description of the Prior Art
The Czochralski (Cz) method is well known in the prior art for growing a silicon single crystal free of dislocations. According to the Cz method, a seed crystal is lowered into contact with a silicon melt, and then pulled out of the melt at a controlled lift rate. The surface tension of the silicon at the melt interface allows molten silicon to be pulled out of the crucible, where it cools and solidifies on the seed, thus forming a single crystal.
One of the inherent deficiencies in growing a crystal by the CZ method is the generation of dislocations in the atomic lattice of the crystal caused by thermal stress. In an effort to eliminate such dislocations, the prior art technology calls for growing a first part of the crystal, known as the "neck section," to a diameter smaller than that of the seed. A typical neck section of the crystal will be reduced to approximately less than or equal to three millimeters in diameter. As the crystal neck is grown, the cross-section of the neck, and the thermal stress related thereto, are reduced, which help to eliminate dislocations in the crystal. The neck continues to be grown until the neck is visually determined to be dislocation free by the operator, upon which the cone and body of the crystal are grown. The diameters of the neck section must be small in diameter (e.g., 2 to 4 millimeters) to eliminate the dislocation generation.
Because the CZ method of growing crystals is very power intensive and very time intensive, productivity measures call for growing crystals as long as possible, being physically constrained only by the dimensions of the pulling machine and the available molten silicon. In addition, as integrated circuit technology increases, the demand for larger diameter crystals increases.
The larger diameter crystals result in increased weight of the grown crystal. This increase in weight becomes a concern, as the entire weight of the crystal must be supported by the neck section of the crystal. Of particular concern is the strength of the neck section at elevated temperatures. The high temperature of approximately .gtoreq.650.degree. C. causes the thermal related strength properties of the silicon to be such that the neck section could stretch or break from plastic deformation, causing a total loss of the crystal being grown. Further, safety related issues arise if the crystal were to break.
One prior art attempt to address the neck section strength problem relating to the growth of large diameter crystals of 300 mm or larger was to maintain the neck section as cool as possible to improve the thermal properties of the silicon. For example, European Patent publication EP-0-671-491-A1 issued to Shin-Etsu Handotai Co. Ltd. discloses a method of increasing the strength of the neck section of the crystal by growing the neck section with lengths between 200 mm and 600 mm. It further calls for restricting the thickness variation of the neck to less than 1 mm, and the minimum diameter of the neck to be .gtoreq.2.5 mm. The increased length of the neck allows the neck to get farther away from the molten silicon and the heaters, thus allowing the neck to cool down and thereby increase the strength properties of the neck. However, although effective in cooling the neck and thus providing strength and eliminating breakage of the neck section, this method causes a loss of productivity due to the need to use 200 mm to 600 mm of the pulling chamber for the neck section, which cannot then be used for ingot fabrication. Another disadvantage of this method is the difficulty in maintaining a thin neck with a restricted thickness variation for an extended length.